1. Field of the Invention
This invention relates to a magnetic optical device which is used in optical communication systems, laser processing systems and so forth. More particularly, it relates to an improvement in a Faraday rotator used in, e.g., an optical isolator by means of which light emitted from a light source and reflected at an end face of an optical element is prevented from returning to the light source.
2. Description of the Related Art
The Faraday rotator is a functional element consisting basically of a Faraday element and a magnet which applies a magnetic field to this Faraday element to produce Faraday effect. It is used in magnetic optical devices such as an optical isolator which is necessary to intercept light returning to a semiconductor laser to stabilize its lasing; the semiconductor laser being used in optical communication systems, laser processing systems and so forth.
FIG. 1 is a sectional view of a fundamental Faraday rotator used at present in optical communication systems. In FIG. 1, reference numeral 10 denotes a permanent magnet (a magnet member) standing magnetized in the optical-axis direction (shown by black arrows), which commonly has the shape of a cylinder and into a through-hole of which a Faraday element 11 is kept inserted. The Faraday element 11 produces the Faraday effect by the aid of a magnetic field formed in the optical-axis direction in the through-hole made in the permanent magnet 10. Conventionally, the wavelength band of light that is used in optical communication systems is in the range of from 1.3 μm to 1.7 μm, and, in some optical isolators, the Faraday rotator has been used in such a form that a rare earth iron garnet film is inserted as the Faraday element into the permanent magnet.
In recent years, there is an increasing demand for optical isolators used to protect semiconductor lasers which are used when fiber lasers for processing machines are excited. The wavelength of light used here is a shorter wavelength than the optical communication band, which is chiefly a wavelength of around 1 μm. Then, at such a wavelength of around 1 μm, the rare earth iron garnet film has too large absorption of light to be durable to its use. Accordingly, a paramagnetic material such as terbium-gallium-garnet (hereinafter simply “TGG” in some cases) or terbium glass is used as the Faraday element.
Especially where the Faraday rotator is used in the optical-isolator, the plane of polarization of light is rotated by the aid of the Faraday effect, and it must be rotated at an angle (hereinafter “Faraday rotation angle”) of 45 degrees (hereinafter also “45°”).
It comes to Faraday rotation angle=V×L×H where the length of a Faraday element is represented by L, the Verdet constant by V, and the magnetic field in the optical-axis direction by H.
The Verdet constant is spatially invariable, but the magnetic field formed by a magnet in the optical-axis direction is not necessarily invariable. Hence, in reality, it comes to:Faraday rotation angle=ΣV·H(L)·ΔL.  Expression (2)
However, the paramagnetic material such as TGG or terbium glass has a small Verdet constant, and its ability to rotate the plane of polarization is so small that it may require a strong magnetic field. As the result, this has made it necessary for the paramagnetic material to be of large size which is long in the optical-axis direction, and at the same time has made huge the permanent magnet as well that magnetizes this paramagnetic material.
Then, making the paramagnetic material and permanent magnet huge not only leads to an increase in cost, but also, because of a strong leak magnetic field, makes it difficult to assemble the optical isolator and further to incorporate the optical isolator in the device.
In addition, where the paramagnetic material has a long size in the optical-axis direction, the paramagnetic material absorbs light to come to be of high temperature when laser light is transmitted therethrough, and this makes the Faraday rotation angle deviate from 45°, also resulting in a lowering of the performance required as the optical isolator.
As a means by which such problems can be resolved, a Faraday rotator has already been developed in which, as shown in FIG. 5, a magnet member having a through-hole is made up of a first magnet 1 standing magnetized in the direction of magnetization that is perpendicular to the optical axis and the direction taken toward the optical axis and a second magnet 2 standing magnetized in the direction of magnetization that is perpendicular to the optical axis and the direction taken against the optical axis, and a Faraday element 5 is disposed inside the through-hole of the magnet member, so as to be improved in magnetic field strength for the Faraday element 5 [see Japanese Patent Laid-open Application No. 2004-302412 (hereinafter “Patent Document 1”) and Japanese Patent Laid-open Application No. 2007-248779 (hereinafter “Patent Document 2”)].
Thus, the means disclosed in these Patent Documents 1 and 2 has enabled improvement in magnetic field strength for the Faraday element. However, in order to make the Faraday rotator more compact, it is sought to develop a stronger structure for the magnet member.
Now, to resolve the problem that the Faraday rotation angle may deviate because of a temperature rise of the paramagnetic material (Faraday element), a method is considered effective in which a plurality of short paramagnetic materials are set in combination so as to improve heat dissipation properties, e.g., a method in which two Faraday rotators that provide a Faraday rotation angle of 22.5 degrees are connected in series.
However, if Faraday rotators as disclosed in Patent Document 1 are connected in series, there is a problem that the magnetic field applied to the Faraday elements may result in a low strength unless a sufficient space is provided between the two Faraday rotators.
A method may also be contemplated in which the structure of the optical isolator disclosed in Patent Document 2 is applied, which has two Faraday rotators. However, this optical isolator disclosed in Patent Document 2 is a device which functions as such in virtue of the structure of what is called a semi-duple type optical isolator in which a polarizer is disposed between Faraday rotators providing a Faraday rotation angle of 45 degrees (such an optical isolator with a polarizer disposed between Faraday rotators is called the semi-duple type optical isolator structure, where Faraday rotators providing a Faraday rotation angle of 45 degrees are used). Thus, if the Faraday rotation angle provided by the two Faraday elements is set at 22.5 degrees under the make-up of the magnets disclosed in Patent Document 2, the device can not function effectively when an optical isolator is made up using them in combination with the polarizer.